ComScore and others have reported Facebook (FB -0.9%) has seen a major growth slowdown in U.S....

ComScore and others have reported Facebook (FB-0.9%) has seen a major growth slowdown in U.S. Web usage, thanks to market saturation and growing mobile usage. That leads Oppneheimer's Jason Helfstein to worry the company will miss Q2 revenue estimates on Thursday: he predicts U.S. ad growth will slow to a mere 7% Y/Y from Q1's 27%, and international growth to 32% from 48%. However, Helfstein admits his estimates don't account for Facebook's nascent mobile ad sales. (Q1 slowdown)